Because of the energy crisis, considerable effort has been expended in recent years to develop improved heating systems for homes and small businesses, and particularly those heating systems which use readily available fuels such as wood. One development has been the use of wood fired boilers which are located outside of the building being heated. Typically, a wood fired furnace or boiler is situated in a small building a distance from the home or business being heated and the heat is then piped underground to the building. This reduces the risk of fire and also reduces the amount of dirt and trash being introduced into the house or business with the wood being burned. Although furnaces and boilers which have been developed for use within the building being heated can be used in this application, there has developed a need for a rugged, easily constructed, relatively inexpensive wood fired boiler which will extract as much heat as possible from the wood being burned and store that heat until it is used.